Python 3.14 vs 3.15: What Changed and Which to Use

Python 3.14 vs 3.15 compared: t-strings, free threading and zstd in 3.14, lazy imports, the faster JIT and UTF-8 default in 3.15, and which version to run.

Run Python 3.14 for production projects with many dependencies, and Python 3.15 for new projects or when you want lazy imports. Both are supported for years, so this is about ecosystem maturity more than about features.

At a glance

What Python 3.14 added

• Template strings (PEP 750): t"..." literals that produce a template object rather than a finished string, so libraries can safely process interpolated values for things like SQL or HTML.
• Deferred evaluation of annotations (PEP 649 and 749): annotations are evaluated lazily, which removes most need for string annotations and from __future__ import annotations.
• Free-threaded builds officially supported (PEP 779): the no-GIL build is no longer experimental. See free threading explained.
• Multiple interpreters in the standard library (PEP 734): concurrent.interpreters for isolated parallelism.
• Zstandard compression (PEP 784): a compression.zstd module.
• Safe external debugger interface (PEP 768).

What Python 3.15 added

All of these are covered in what is new in Python 3.15:

• Lazy imports (PEP 810): defer a module import until first use, cutting startup time for CLI tools.
• frozendict as a built-in immutable mapping.
• A profiling package including the Tachyon sampling profiler.
• UTF-8 as the default encoding (PEP 686).
• A faster experimental JIT. See the Python 3.15 JIT.

Which should you run?

• Existing production service: 3.14, unless a feature in 3.15 solves a problem you have. Your dependencies’ wheels and your CI images are ready for it.
• New project: 3.15 is reasonable if the packages you need already ship wheels for it; check before committing.
• Library maintainer: test and publish for both. Users move at different speeds.
• Command-line tools where startup time matters: 3.15’s lazy imports are worth testing.

Whichever you pick, start on a later patch release (3.15.2 or later, say) rather than the first .0.

Upgrading from 3.14 to 3.15

Read Porting to Python 3.15 for what was removed or changed. The UTF-8 default is the change most likely to alter behaviour: open() without an encoding argument now reads and writes UTF-8 whatever the system locale, which mostly fixes Windows surprises but can change how you read legacy files.

For dates across all versions, see the Python end-of-life dates page. If you are still on 3.12 or older, the 3.12 end-of-life guide explains the move.